1. Field of the Invention
The invention relates to a method for replacing a secondary cell in a case where one or more secondary cells of a battery pack formed by electrically connecting a plurality of secondary cells in series or parallel have deteriorated, or have come to an end of service life, or have failed. The invention particularly relates to a method for replacing a secondary cell installed in an electric vehicle (PEV) or a hybrid vehicle (HEV).
2. Description of the Related Art
Recently, many electric vehicles (PEVs) and hybrid vehicles (HEVs) having both an electric motor and an internal combustion engine employ nickel-hydrogen (Ni-MH) secondary cells as a major electric power supply for driving an electric motor, for the high energy density (i.e., compact storage of energy) and the high output density of the nickel-hydrogen (Ni-MH) secondary cells. In order to supply sufficient power to an electric motor, such PEVs and HEVs adopt a battery pack formed by combining a plurality of mono-block type battery modules in which a plurality of unit cells are internally connected.
With regard to such Ni-MH secondary cells installed in PEVs and HEVs in the form of a plurality of connected battery modules, a long service life is possible if the environment of use is appropriate. In most cases, however, the battery modules individually come to have an abnormality or reach an end of service life one at a time due to individual differences of secondary cells or defects in component parts of secondary cells. The battery module that has been judged as an abnormal module or has come to an end of life is replaced with a new battery module, and the thus-refitted battery pack is used.
However, in the case where a battery module that has been judged abnormal or has come to an end of life is removed from a battery pack made up of a plurality of battery modules and a new battery module is incorporated, problems as stated below occur due to characteristic differences between the battery module newly incorporated (hereinafter, referred to as “new battery module”) and a battery module that has been and is still in use in the battery pack (hereinafter, referred to as “old battery module”).
(1) As indicated in FIG. 5, during a vehicle run, the voltage difference (ΔV) between the voltage value (V60) of the new battery module and the voltage value (V61) of an old battery module may become large, so that an electronic control unit of the battery (hereinafter, referred to as “battery ECU”) may possibly make a false detection of a “voltage dispersion abnormality” to protect the battery.
(2) As indicated in FIG. 6, as the remaining amount of charge (SOC: state of charge) of each battery module is computed, the difference (ΔSOC) between the SOC value (SOC70) of the new battery module and the SOC value (SOC71) of an old battery module may become large, so that the battery ECU may possibly make a false detection of an “SOC dispersion abnormality” to protect the battery.
The above-stated problems are caused as follows. That is, an old battery module, having a history of use, has stored a memory effect. Therefore, as indicated in FIG. 7, the old battery module has a voltage characteristic (V81) that does not coincide with the voltage characteristic (V80) of a new battery module incorporated at the time of replacement.